The average fuel economy in a newly sold vehicle in May showed no change from either the previous month or the previous year, according to the University of Michigan Transportation Research Institute.

The typical vehicle sold in the United States had a combined city and highway fuel economy of 25.3 miles per gallon. This was the same value recorded in both April and May 2016.

UMTRI researchers Michael Sivak and Brandon Schoettle have been tracking changes to the sales-weighted fuel economy in light vehicles—including cars, SUVs, trucks, and vans—for nearly 10 years. The values are determined based on monthly sales figures as well as the fuel economy for individual models recorded in the Environmental Protection Agency's "Fuel Economy Guide."

When the study began in October 2007, the average vehicle sold in the U.S. had a combined fuel economy of 20.1 miles per gallon. The highest average was recorded in August 2014, when the typical vehicle purchased during the month got 25.5 miles per gallon.

With gas prices staying considerably lower than 2014 levels, the average sales-weighted fuel economy have shown little change since the record high. Each month's figure has varied between 24.8 miles per gallon and 25.3 miles per gallon. However, May marked the fifth month where the fuel economy average has equaled or exceeded that of the previous month.

The model year average, which measures the average fuel economy over a year between October and September, improved steadily from 20.8 miles per gallon for the 2008 model year to 25.1 miles per gallon for the 2014 model year. It stayed at 25.1 miles per gallon for the 2015 and 2016 model years.

Sivak and Schoettle also measure an "Eco-Driving Index," which estimates the level of emissions generated by individual drivers of new drivers as compared to new vehicle drivers in October 2007. This figure is updated on a two-month delay due to a corresponding lag in underlying data from the Federal Highway Administration.

The Eco-Driving Index has worsened or showed no improvement each month since August 2016, when it stood at 0.80. The figure for March was 0.85, indicating that the emissions generated by an average new vehicle driver during the month were 15 percent lower than in October 2007.

The researchers also determined that drivers in March tallied up 5 percent more vehicle miles than in October 2007. For the 10th consecutive month, the fuel used per distance driven was 19 percent lower than baseline levels.